Field of the Invention
The present invention relates to a method for continuously casting a slab containing titanium or a titanium alloy
Description of the Related Art
Conventionally, an ingot has been continuously cast by melting metal by vacuum arc or electron beam, and pouring the metal into an open mold where the metal is solidified and withdrawn from the bottom of the mold.
An ingot containing titanium or a titanium alloy is continuously cast while the surface of the molten metal in the mold is heated by plasma arc or electron beam.
If an excessively high heat input is applied to the surface of the molten metal in the mold, a solidified shell is not sufficiently grown and becomes excessively thin. Thus, when the solidified metal is withdrawn, the surface of the solidified shell is torn due to lack of strength, which leads to an accident such as bleed-out. In contrast, if an excessively low heat input is applied to the surface of the molten metal in the mold, a solidified shell is overgrown, resulting in spread of the molten metal. This leads to a large surface defect and makes it impossible to assure a sufficient molten metal pool, which precludes continuous casting. Thus, the amount of heat input should be in a proper range for good cast surface quality.
When a slab having a rectangular cross-section is continuously cast, there is a limit to the size of a chamber for accommodating a casting machine, and the molten metal is typically poured from a hearth into a mold through one of the paired shorter sides of the rectangular mold. However, the flow and the temperature of the molten metal create a difference in the temperature of a region near the surface of the molten metal between the metal inlet side and the side opposite the metal inlet side, and heat input is applied circumferentially non-uniformly. As a result, the solidification varies with circumferential position in a slab, which degrades the cast surface quality of the resulting slab.
A slab with poor cast surface quality requires removal of surface flaws before rolling, causing problems such as decreased yield and increased operations, which are responsible for increased cost. Thus, there exists a need for casting a slab with its cast surface having minimum irregularities and flaws.
JP 2013-107130 A discloses a method for casting a titanium slab to be hot rolled, the method including pouring molten metal simultaneously from the both walls on the paired shorter sides of a mold. Pouring of molten metal simultaneously from the both walls on the paired shorter sides ensures uniform temperature of the molten metal in the mold along the length of the mold walls on the opposing longer sides, which suppresses deformation (warpage) in the thin thickness direction. The temperature is also uniform along the length of the mold walls on the shorter sides, which can further inhibit deformation (bending) in the width direction.
JP 2014-233753 A discloses a method for melting and re-solidifying the surface of an ingot prepared by casting an ingot and cold-working the surface layer of the ingot or only by melting metal and casting an ingot. Melting and re-solidification of only the surface layer of an ingot allows provision of a pure titanium ingot for industrial use with decreased surface flaws and good surface quality.